Seamless UV Texture?
Split your view, so that you can have 3dview and UV Window.
Select Texture Paint in 3d view.
In Uv window, Turn on N Panel and in Display check Repeat.
This alows you to tile your texture.

Now in 3dview, select Clone tool. Choose a point in your textures, which you want to clone. Left click to move the curson at chosen point (Ctrl+Left CLick)  and start cloning your texture to seamless one.

Move your UV layout in uv window, when you want to offset the texture.